The disclosure relates to personal computer systems generally and, more particularly, to an audio amplifier arrangement that selectively switches between voltage supplies, the selection depending on the computer user's election to avail himself of a particular one of two or more available speaker systems that may include, for example, either the personal computer's internal speakers or externally connected headphones.
Computer systems in general, and personal computer (PC) systems in particular, have attained widespread use in providing computer power to many segments of modern society. A conventional PC system can usually be defined as a desktop, floor standing, or portable microcomputer that includes a system unit having a system processor and associated volatile and non-volatile memory, display monitor, a keyboard, one or more diskette drives, a fixed disk storage device and an optional printer. One of the distinguishing characteristics of these systems is the use of a system board to electrically interconnect these components. PC systems may be considered information handling systems that are designed primarily to provide independent computing power either to a single user or to a relatively small group of users, as in the case of personal computers that serve as computer server systems. Accordingly, such systems are intended to be inexpensively priced for purchase by individuals or small businesses. A PC system may also include one of a plurality of peripheral or I/O devices that are coupled to the system processor and that perform specialized functions. Examples of I/O devices include modems, sound and video devices or specialized communication devices. Mass storage devices, such as hard disks, CD-ROM drives and magneto-optical drives, are also considered to be peripheral devices. Computers producing multi-media effects, i.e., sound coupled with visual images, are in increased demand as computers become used for artistic endeavors, for entertainment, and for education. In addition, the use of sound makes game playing more realistic and helps reinforce knowledge and make educational programs more enjoyable to use. Digital effects and music can also be created on the computer and played through attached speakers without the need for additional musical instruments or components.
Of course, as the audio aspect of a personal computer's operation acquire increasing significance, the design and operation of the computer's audio system, which may be taken to include, at least, audio storage and playback components, audio amplifiers, and audio outputs (speakers), attracts concommitantly greater attention. As with other aspects of the computer design, design of the audio system is confronted with the often competing considerations of performance and expense. In addition, the PC audio amplifier must exhibit both quality and flexibility of performance.
Specifically, audio amplifiers, such as those used in a majority of portable computer platforms, are configured to drive both the high-power internal speakers in bridged-mode operation, as well as the low-power, noise-sensitive, headphone/line output paths in single-ended mode. In the bridged mode (also sometimes referred to as “balanced” mode), an audio speaker is typically coupled across two differential outputs of an audio amplifier. The bridged, or balanced, configuration has a number of advantages, salient ones of which are the maximization of available voltage swing across the speaker and the rejection of common-mode signals, such as noise or DC offsets. In the single-ended configuration, the audio amplifier provides a single output, and the speaker is (usually AC) coupled between the amplifier output and a reference potential, usually ground (GND). Because, as suggested above, PC systems accommodate both internal speakers and headphones, the audio system designer is required to elect to support both the modes equally, or to allocate a performance preference to either the bridged mode or the single-ended mode. Inasmuch as the audio amplifier operating requirements applicable to use of internal speakers are dissimilar to those applicable to the use of headphones, the design challenges are not trivial. For example whereas internal speakers may be expected to present to the amplifier an impedance of a few ohms, a headphone jack may present an impedance of several ohms up to thousands of ohms. Therefore the amplifier must be designed to supply greater current levels to internal speakers then to headphones. The current demanded from the audio amplifier power supply varies proportionally to the output power required from the amplifier. Simultaneous support of both modes, while maintaining noise immunity and optimal audio performance, necessarily is attended by increased system cost and complexity. Alternatively, if either mode is afforded preference over the other, then compromises must be encountered. A preference for the single-ended mode will result in excessive power losses in the bridged mode; conversely, a preference for the bridge mode will result in exacerbated power supply noise in the single-ended mode.
In the high-power bridged mode, the power supply is required to deliver large amounts of peak power to the audio amplifier. Therefore, to prevent unwanted power loss in this high peak current configuration, the supply path is optimized to eliminate, or at least minimize, series impedances. AC noise from the power supply is a less significant concern in the bridged mode because noise voltage appearing at the output of the power supply will be common to both the positive and inverting output stages and will, therefore, effectively cancel.
In the low-power single-ended mode, the ability to effectively handle large peak currents is no longer a concern, and focus is directed to power supply noise immunity. Unlike the bridged mode, in the single-ended mode, noise that propagates to the output stage via the power supply path will not be canceled and will appear as a differential voltage at the output. In addition, typical low-power, single-ended configurations, such as headphones and external speakers, represent situations in which output noise is more perceptible to the end user, and is therefore more objectionable.
System designers have exploited various methods of providing power supply noise immunity in high-current configurations. In one approach, the audio system power supply is optimized for peak current-handling capability. Only passive noise immunity, usually in the form of bulk or small bypass capacitance filtering of the rectified voltage derived from the AC power line, is provided. This approach is typified by the circuit configuration depicted in FIG. 1, which represents a single channel of a typical stereo implementation.
As may be seen in FIG. 1, the audio system of a PC may be generally represented as including an audio amplifier 10 coupled through an input capacitor C1 to an input signal. (The source of the input signal to audio amplifier 10 is not considered an aspect of the invention herein.) A typical implementation of a bridge-type audio amplifier 10 includes both an inverting amplifier 11 and a noninverting amplifier 12. The input signal, which may be provided by a preamplifier (not shown), is coupled both to input 121 of noninverting amplifier and to input 111 of the inverting amplifier. Output 122 of the noninverting amplifier is coupled to one end of speaker 20, and output 121 of inverting amplifier 11 is coupled to the opposite end of speaker 20. In this manner, speaker 20 is provided a balanced signal from amplifier 10, and the amplifier is said to operate in a balanced, or bridged, mode.
As shown in FIG. 1, amplifier output 112 may be coupled to speaker 20 through a switch SW1 that is driven by a jack-sense indicator 50. In general, jack-sense indicator 50 operates to detect the insertion of headphones, or an equivalent ancillary speaker system, into jack 40. Many implementations of jack-sense indicator 50 are available to those acquainted with the art. For example, jack-sense indicator 50 may simply operate to detect the impedance to GND present at jack 40. If headphones are not connected, then the detected impedance is high, and may be taken to approach infinity. If headphones are connected, then the impedance at jack 40 will be below some nominal predetermined threshold, say 100 Kohms. Jack-sense indicator 50 then responds by providing a logic-level signal at its output. In an exemplary embodiment, jack-sense indicator may provide a logic-level ONE when headphones are connected, and logic-level ZERO otherwise. Alternatively, jack-sense indicator 50 may be integrated into one device with audio jack 40. An example of such a device is commercially available from Foxconn International, Inc., Sunnyvale, Calif. In this approach, jack-sense indicator 50 is simply mechanical switch that is driven by headphones that are inserted into jack 40. The switch may be wired so that it connects to GND when no headphones are inserted, and connects through a pull-up resistor to a DC voltage (logic-level ONE) when headphones are connected.
The binary output of jack-sense indicator 50 is coupled to SW1 and determines the condition (open, closed) of SW1. The amplifier design arbitrarily assumes that headphones are not connected, so that amplifier output 112 is normally connected (that is, SW1 is normally closed) to speaker 20 through speaker connection 22. With SW1 closed, amplifier output 112 is connected to speaker 20. However, when headphones are connected to headphone/line out jack 40, jack-sense indicator causes SW1 to be driven open, and a signal path between amplifier 10 and speaker 20 is interrupted. Specifically, the connection between amplifier output 112 and speaker 20 is broken by SW1. Nevertheless, the connection from amplifier 10 to jack 40 is maintained from output 122 of noninverting amplifier 12 through a coupling capacitor C2. In this configuration, amplifier 10 is said to operate in the single-ended mode.
The operation of amplifier 10 is enabled by a voltage supply system 30. Voltage supply system 30 is shown to include a DC voltage source, VDD, that is derived from the AC power line (not shown). Voltage source VDD may, in fact, be generated simply by rectification of the AC power line, and by filtering the rectified voltage with a line filter capacitor. (It should be recognized, however, that the invention is not predicated on the manner in which VDD is generated.) Voltage source VDD is coupled to a voltage supply node 13 associated with amplifier 10 and is coupled from node 13 to both amplifier 11 and amplifier 12. In addition, voltage supply system 30 is shown also to include an undesired figurative noise source, VN, that operates additively with respect to VDD. For the pedagogical purposes of this Description, VN is intended to represent noise signals that may derive from any one or more of a number of ambient sources, or may represent the manifestation of a number of phenomena. For example, VN may represent unfiltered random noise that is present on the AC line. Additionally, VN may represent unfiltered ripple at 60 Hz, or some harmonic thereof, that results from rectification of the AC line. Further, VN may represent a contaminant signal of external origin, such as noise generated by the PC hard disk system. The point, however, is that VN represents a spurious, and undesired, signal that is ultimately coupled to voltage supply node 13.
It is readily appreciated, therefore, that the arrangement of FIG. 1 results in increased susceptibility to (especially low-frequency) power supply noise in the single-ended mode. Consequently, noise present at the amplifier voltage supply node 13, including but not limited to noise generated by the hard drive or power supply hum, will be coupled to, and therefore readily perceptible on, the headphone and line output paths.
An available response to the above-described problem may be had by the inclusion of a voltage regulator in series with the power amplifier supply path. Although this approach does provide some rejection of the random noise, VN, present on the VDD line, it is accompanied by a significant increase in system size and cost. In addition, the voltage regulator will necessarily limit output power in the bridged mode as a result of the attendant series voltage drop and likely current limitations.
Accordingly, what is desired is an audio configuration for a PC that tends to optimize performance in both the bridged mode as well as in the single-ended mode. In bridged mode, the power supply should satisfy high peak-power requirements. In the single-ended mode, the power supply should afford substantial noise immunity. The resulting design preferably provides the desired performance without serious penalties in area of system cost and complexity. In addition, the solution should be amenable to implementation through convential integrated circuit design techniques.